1. Field of the Invention
The present invention relates to pressing means for pressing a plate portion at the time of working using various welding apparatuses or a laser beam machining apparatus, and an industrial robot.
2. Description of the Prior Art
When plate portions are welded, a gap between two lapped plate portions has an influence on the weld quality. Therefore, a method is known in which welding is performed while plate portions are pressed by a pressure roller or the like in order to correct the floating of plate portion so that the gap is kept constant.
For example, Japanese Patent Application Laid-Open No. 8-90264 has discloses a method in which welding is performed while plate portions to be welded are pressed by a roller from the upside of the lap joint in laser welding. FIG. 14 shows a method for laser welding. In this method, a bracket 67 attached to the distal end of a robot arm 100 is provided with a laser welding nozzle 64 used as a working tool and an air cylinder 60 used as a driving source for pressing plate portions, which are welded parts of workpieces 1a and 1b. A roller support frame 61 for supporting a roller is attached to a slider, which is a movable part of the air cylinder 60, and a roller 62 is rotatably mounted at the distal end of the roller support frame 61. The roller 62 can be displaced vertically by the extending/retracting operation of the air cylinder 60. When laser welding is performed, the air cylinder 60 is operated to press the roller 62 on the plate portions of the workpieces 1a and 1b to be welded by the welding nozzle 64 in the vicinity of the weld portion. Thus, laser welding is performed along a weld line while the floating of plate portions of the workpieces 1a and 1b is corrected so that the gap is regulated.
Also, there is publicly known a construction in which a damper 66 such as a coil spring is disposed in place of the air cylinder 60 so that a constant pressing force is applied to the roller 62 by the damper 66 as shown in FIG. 15 (see Japanese Patent Application Laid-Open No. 9-327781).
In the case where a coil spring is used to control the pressing force of the roller, the pressing force cannot be controlled, and various kinds of plate portions of workpieces with different shapes and plate thicknesses cannot be pressed by one type of pressing device. In order to press such various kinds of plate portions, it is necessary to replace the spring with one having a different spring constant.
Also, as described above, in the case where an air cylinder is used to control the pressing force of roller, only a fixed pressing force can be given because a general electromagnetic valve can merely turn on or off air, so that various kinds of plate portions cannot be pressed as in the case of the coil spring. Even in case of using the air cylinder, pressure can be controlled by using a proportional control valve to regulate the pressing force. In this case, however, there arises a problem in that the response is slow, so that it is difficult to delicately change the pressing force during high-speed welding and working.
Also, the conventional construction is such that only a damper is disposed so as to respond to variations in distance between the working tool such as a welding head and the plate portion, and the roller pressing direction cannot be changed when the angle of the working tool such as a welding head with respect to the plate portion changes. Therefore, if the angle of the working tool such as a welding head with respect to the plate portion changes, the pressing force changes accordingly, which presents a problem in that high-quality working including welding cannot be performed.